Aromatic polyesters typified by polyethylene terephthalate and polyethylene-2,6-naphthalene carboxylate are widely used in films because they have excellent mechanical properties, dimensional stability and heat resistance. Especially polyethylene-2,6-naphthalene dicarboxylate has more excellent mechanical properties, dimensional stability and heat resistance than polyethylene terephthalate and is used in base films for high-density magnetic recording media which require dimensional stability. However, the requirement for dimensional stability in the high-density magnetic recording media is recently becoming higher and higher and the further improvement of their characteristic properties is desired.
Patent Documents 1 to 4 propose polyalkylene-6,6′-(alkylenedioxy)di-2-naphthoates obtained from a diethyl-6,6′-(alkylenedioxy)di-2-naphthoate which is an ester compound of a 6,6′-(alkylenedioxy)di-2-naphthoic acid. These documents present crystalline polyethylene-6,6′-(ethylenedioxy)di-2-naphthoates having a melting point of 294° C.
However, as the polyethylene-6,6′-(alkylenedioxy)di-2-naphthoates disclosed by these documents have a very high melting point and very high crystallinity, they have low fluidity in a molten state, making extrusion nonuniform at the time of forming a film, or their films are broken due to crystallization when they are stretched at a high draw ratio.
Patent Document 3 teaches that a magnetic recording flexible disk having little track slippage is obtained from a film of polyethylene-6,6′-(ethylenedioxy)di-2-naphthoate having a maximum temperature expansion coefficient of 10 to 35 (ppm/° C.), a maximum humidity expansion coefficient of 0 to 8.0 (ppm/% RH), a difference between the maximum and minimum temperature expansion coefficients of 0 to 6.0 (ppm/° C.) and a difference between the maximum and minimum humidity expansion coefficients of 0 to 4.0 (ppm/% RH).
In an Example of the above document, there is disclosed a film having a Young's modulus in the film forming direction of 485 kg/mm2 (4.8 GPa) and a Young's modulus in the crosswise direction of 1,100 kg/mm2 (10.9 GPa), a maximum temperature expansion coefficient of 19 (ppm/° C.), a minimum temperature expansion coefficient of 16.5 (ppm/° C.), a maximum humidity expansion coefficient of 6 (ppm/% RH), and a minimum humidity expansion coefficient of 4.5 (ppm/% RH).
However, since the requirement for the improvement of recording density in magnetic recording media is now very severe, and dimensional stability required for a base film cannot be attained by polyethylene terephthalate, polyethylene-2,6-naphthalene dicarboxylate and the film proposed by Patent Document 3.
More excellent surface properties such as traveling property and abrasion resistance are desired.
    (Patent Document 1) JP-A 60-135428    (Patent Document 2) JP-A 60-221420    (Patent Document 3) JP-A 61-145724    (Patent Document 4) JP-A 6-145323